part -A] GEOLOGY OF COKR1S AND ABERLLEEEIWI. 523 



The upper and lower limits of this slate-band (or, as the quarry- 

 men term them, the 'top slip' and 'bottom slip') are, in nearly 

 every case, very well defined. The lower limit is clearly marked 

 off by its contact with the somewhat rubbly, rusty-weathering 

 mudstones of the Red Vein ; while the top of the slate-band ends 

 off abruptly against the grits and gritty mudstones of the Garnedd- 

 wen Beds. At Aberllefenni particularly, the slate-band terminates 

 against a massive grit-band quite abruptly. The slate is stripped 

 off this grit-band, and the latter then constitutes a good ' roof ' in 

 underground workings. North-east of Aberllefenni Quarries (for 

 instance, at Katgoed, where the slate-band is overlain by mud- 

 stones), the upper limit is perhaps not capable of such precise 

 definition as farther south ; but, even then, there is a well-marked 

 lithological change within a few feet. However, in practically all 

 the quarries, the full width of the slate has been worked, and the 

 band is correspondingly well exposed. 



There seems to be little doubt that the physical characters of 

 the Narrow Vein vary from point to point and within compara- 

 tively short distances, although such changes cannot be observed in 

 hand-specimens. These changes have important effects upon the 

 economic value of the slates and slabs obtained from it. One of 

 the chief difficulties against which quarry-owners have to contend, 

 is the tendency for slabs to 'pillar' — that is, to split at right angles 

 to the cleavage and in directions opposed to the jointing. Slabs 

 from some quarries, even in this comparatively narrow slate-band, 

 appear to 'pillar' more readily than those obtained from others. 

 even when the quarries are separated by quite a short distance. 

 In fact, I believe, slabs obtained from different parts of the same 

 quarry have somewhat different physical properties. The tendency 

 to ' pillar ' is especially noticeable when the slabs are exposed to 

 the atmosphere for some time, or experience sudden changes of 

 temperature. The exact cause of ' pillaring,' etc. is, so far as I am 

 aware, at present imperfectly understood. 



In addition to roofing-slates, the Narrow Vein yields large slabs 

 of excellent quality, which are marketed for a variety of purposes : 

 as, for example, billiard-tables, brewing-vats, electric switch- 

 boards, etc. 



The band is about 50 to 60 feet thick, sometimes rather less 

 than that amount : for instance, in the Gaewern Quarry. 



I have not obtained any fossils from the Narrow A T ein, but 

 Mr. Gr. J. Williams has collected Orthoceras perannulatum from 

 the Braich-goch Quarries and Phacojjs sp. from the Cymerau 

 Quarries. 



(%) Garnedd-wen Beds. — These beds succeed the Narrow 

 Vein conformably ; they consist of dark-blue micaceous mud- 

 stones, often sandy or gritty, and contain subordinate bands of 

 massive grit. 



The mudstones possess very marked lithological characters, and 

 may readily be distinguished from all other sedimentary types 



